The invention relates to a process for producing a polypropylene film having a small dynamic coefficient of friction (CoF) and to a film obtained by this process. The invention further relates to the use of a polypropylene composition in a film forming process in order to reduce the required amount of slip agent for achieving a certain CoF and, alternatively, to the use of a polypropylene composition in a film forming process in order to reduce the required storage time for achieving a certain CoF.
Castfilm made of polypropylene has the disadvantage of very slow slip agent migration compared to e.g. polyethylene. Especially castfilms made of visbroken polypropylenes with a thickness of below 50 μm do after production not have the small CoF which is required for a good winding quality and also for the following conversion steps. For a good winding quality a CoF of approx. 0.35 (depending on film properties and film thickness) and for conversion a CoF below 0.2 is technically necessary and therefore a market requirement.
In order to fulfill this requirement it is state of the art to add slip agent (in combination with antiblocking agent) to the polypropylene cast film grades. The slip agent migrates to the surface of the film and creates a slippery film which lowers the CoF dramatically. Since the migration to the surface of the slip agent in polypropylene is very slow, an excessive amount of slip agent is added in order to overcome this problem and to obtain the required CoF immediately after film production for good winding quality. If less slip agent is added the CoF immediately after film production is too high and the film has to be stored for a long period (ca. 14 days) in order to achieve a sufficiently small CoF.
The disadvantages of this solution are                high costs due to excessive amount of slip agent        deterioration of optical film properties after a longer period of time because too much slip agent migrates to the film surface.        deposits of slip agent at the castfilm production line when films having a thickness of about 70 μm are produced.        
It is therefore the object of the present invention to provide a novel process for producing a polypropylene film, by polymerising propylene and optionally ethylene and/or one or more C4-C10 α-olefins in the presence of a Ziegler-Natta catalyst to produce a polypropylene and subjecting the polypropylene to a film forming process. The produced film shall have a small dynamic coefficient of friction (CoF) (determined according to DIN 53 375, 24 hrs after production of film) compared to polypropylene films obtained from other processes (but containing—for reasons of comparability—the same amount of slip agent).